1. Field of the Invention
The present invention relates to an aluminum alloy piping material for automotive tubes. More specifically, the present invention relates to an aluminum alloy piping material for automotive tubes having an excellent corrosion resistance and formability that can be suitably used for a tube connecting an automotive radiator and heater, or for a tube connecting an evaporator, condenser, and compressor, and a method of manufacturing the same.
2. Description of Background Art
A pipe used for connecting an automotive radiator and heater or connecting an evaporator, condenser, and compressor is usually expanded at the tube end by bulge forming and connected with a radiator, heater, evaporator, condenser, or compressor. A tube connected with a radiator or the like is connected with a rubber hose and fastened by a metal band. Conventionally, a single pipe made of an Al—Mn alloy such as AA3003 alloy or a two-layer or three-layer clad pipe in which an Al—Mn alloy as a core material is clad with a sacrificial anode material made of an Al—Zn alloy such as AA7072 alloy is used as a piping material.
A piping material made of an Al—Mn alloy tends to develop pitting corrosion or intergranular corrosion when used under severe conditions. When such a piping material is connected with a rubber hose, crevice corrosion occurs underneath the rubber hose, i.e. on the outer surface of the piping material. Occurrence of pitting corrosion and crevice corrosion can be prevented by using a clad pipe. However, such a measure has the drawback of bringing about a substantial cost increase.
As a solution for the above-described problems, there has been proposed a piping material in which Cu and Ti are added to an Al—Mn alloy, while limiting the Fe and Si content to specific ranges so that the alloy has improved crevice corrosion resistance (Japanese Patent Application Laid-open No. 4-285139). This piping material demonstrated satisfactory characteristics under various use conditions. However, this piping material occasionally suffered from insufficient formability in bulge forming of the tube end, or encountered a problem relating to corrosion resistance when exposed to a severe corrosive environment.
The present inventors have, in the course of research to elucidate the problems of insufficient formability and corrosion resistance exhibited by the above Al—Mn alloy piping materials, found that the reduced corrosion resistance is caused by microgalvanic_corrosion_occurring_between the alloy matrix_and various intermetallic compounds existing in the matrix, and also that the dispersion condition of intermetallic compounds affects the formability of the tube end. Based on the above findings, the present inventors have proposed an aluminum alloy as a piping material having excellent corrosion resistance and formability, such an aluminum alloy comprising, in mass percent, 0.3 to 1.5% of Mn, 0.20% or less of Cu, 0.06 to 0.30% of Ti, 0.01 to 0.20% of Fe, and 0.01 to 0.20% of Si with the balance being aluminum and unavoidable impurities, characterized in that, of the Si-based compounds, Fe-based compounds, and Mnbased compounds existing in the matrix, the number of compounds having a diameter of 0.5 μm or more is 2×104 or less per square millimeter (Japanese Patent Application Laid-open No. 2002-180171).
However, the aluminum alloy piping material described in Japanese Patent Application Laid-open No. 2002-180171 still produces occasional cracking at the tube end when the tube end is expanded by bulge forming in actual applications. Therefore, the present inventors have conducted further experiments and studies in an attempt to resolve such problems, and have found that cracking at the tube end is ascribable to an aggregate of Ti-based compounds formed in the alloy matrix and acting as a starting point of the cracks.
The present invention has been made based on the above findings, and an object of the invention is to provide an aluminum alloy piping material for automotive tubes having better formability than the material offered in Japanese Patent Application Laid-open No. 2002-180171 as well as superior corrosion resistance under a severe corrosive environment, and a method of manufacturing the same.